Spinal disorders such as degenerative disc disease, disc herniation, osteoporosis, spondylolisthesis, stenosis, scoliosis and other curvature abnormalities, kyphosis, tumor, and fracture may result from factors including trauma, disease and degenerative conditions caused by injury and aging. Spinal disorders typically result in symptoms including pain, nerve damage, and partial or complete loss of mobility.
Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. In one type of system, an elongated rod is disposed longitudinally along the length of the spine or several vertebrae of the spinal column. The rod may be bent to correspond to the normal or desired curvature of the spine in the particular region being instrumented. For example, the rod can be bent or angled to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along the length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra. For instance, one such fixation element is a hook that is configured to engage the lamina of the vertebra. Another type of fixation element is a spinal screw, which can be threaded into various aspects of the vertebral bone, such as the pedicle.
In one typical procedure utilizing a bendable, angled or linear rod, one or more of the rods is situated on one or both of the opposite sides of the spine or spinous processes. A plurality of bone screws is threadingly engaged to several vertebral bodies, such as to the pedicles of these vertebrae. One or more of the bone screws are maneuvered to manipulate the position or orientation of the vertebral body or bodies to which the bone screw is engaged. The rod(s) are connected or affixed to the plurality of bone screws to apply and maintain corrective and stabilizing forces to the spine.
The bone screws or anchors in spinal procedures can have receivers with channels for the elongated rod or other member that, in some bone anchors, open upward, e.g., directly away from the bone to which the anchor is attached. Other bone anchors utilize channels that open along the medial or lateral side of the anchor to receive the rod. Frequently, after implantation, the bone receiver and bone anchor become loose and move in unwanted directions relative to each other and relative to other components of the spinal implant. In some procedures it would therefore be desirable to utilize a bone anchor where the bone anchor interface with the receiver is fixed relative to one another so that the forces applied to the receiver are effectively transferred to the vertebrae.